The use of large-screen projection systems is well known. They serve different broad areas of application such as e.g. electronic presentations for business, education, advertising, entertainment, simulation and status and information. In these applications, the demand on flexibility of these systems is continuously increasing. Typically, it is expected that projections can be made at different positions and in different sizes. Shifting of the image to be projected is one of the standard requirements for today's projection systems. Furthermore, in portable projectors, for which the projection conditions continuously change, continuous adjustment needs to be allowed.
Furthermore, especially in the case of portable projectors but also for fixed projection systems, adjustable Scheimpflug correction often is an additional demand, as the position and the size of the projected image and thus also the angle of projection often needs to be varied using the same projection system.
Projection lenses typically have a very wide aperture, but a limited focussing depth. The wide aperture allows a lot of light through so that bright images can be projected, but the down side of the projection lenses is that object, image and projection lens need to be on axis to obtain a focussed image from edge to edge. A projection lens typically is aimed down if the projector is ceiling mounted and up if table mounted. These projection angles are non-perpendicular to the display surface or screen and would normally produce an unfocussed image on the screen—at least in a part of the image. This can be corrected by providing Scheimpflug correction. The correction is done by creating an angle between the object and the lens which cancels out the angle between the lens and the image. Thus, Scheimpflug is a correction which allows the image projected by a large screen projector to be uniformly focused from top to bottom and left to right, even though the lenses are not perpendicular to the screen. In modern projection systems, the need for Scheimpflug correction often is very important.
Electrical adjustment of the position of the image, as typically done in cathode ray tubes, is difficult to adopt in for projection systems. For projection systems typically the projection lens is positioned such that it can be shifted. In the earliest systems, this was obtained by providing a double set of rails, whereby the projection lens could be shifted horizontally and vertically in a plane perpendicular to the optical axis of the system. The shifting mechanism then comprises a rail system with a double set of rails and a housing leading to a quite complex structure. Furthermore the rails need to be very firm to support the projection lens.
The requirements for modern projection systems increase continuously, the size of the projection systems also increases significantly and the size and thus the weight of the projection lens also increases. Therefore a firm, compact lens shift mechanism needs to be provided, allowing movement of the lens according to several degrees of freedom. Several shift mechanisms are already known.
U.S. 2003/0095337 A1 describes an adjustable lens assembly suitable for focussing or zooming, as applicable to a camera, camcorder or surveillance system. The adjustable lens assembly described provides the possibility for motorized movement along the optical axis of the system. U.S. 2003/0095337 A1 furthermore describes a system that allows compensation for unwanted tolerances in manufacturing or assembly of the gear shaft or the guiding elements. By providing a ball-joint-like connection between the lens and the driving member for the linear movement along the optical axis of the system, the bias in the different directions can be absorbed and the movement of the lens can be smooth.
With projection lenses of a larger projection system, the weight of the projection lens will become a limiting factor in to allow the use of ball joint connections. Furthermore, the projection lens should not only be able to move according to different degrees of freedom with respect to the remaining part of the projection system under influence of bias, there should be furthermore also a possibility to obtain the position and/or orientation with respect to the remaining part of the projection system. The teachings of U.S. 2003/0095337 cannot be applied as such to the problem of fixing and adjusting the position of a projection lens in a large projection system.
U.S. 2002/0067552 describes a projection lens shifting mechanism which allows adjustment of the position of the projected image by translating the optical axis of the projection lens unit. The mechanism is based on two additional plates wherein cams can be moved. The movement of the cams allows a shift of the projection lens in a direction perpendicular to the optical axis of the projection lens unit. As the movement of the cams corresponds with a smaller shift of the projection lens, the position of the projection lens can be determined with high precision. U.S. 2002/0067552 does not describe a way for shifting the projection lens in a motorized way. Furthermore, in order to fix the position of the projection lens, additional screws need to be fastened and a complex system needing multiple plates is needed to obtain the shifting mechanism.
A similar system is provided in Japanese patent JP 2002323648 whereby the mechanism for moving the projection lens in a projector comprises two eccentric plates. By rotating these plates, a free lens shift can be realized. The system can be motorized, using a servo mechanism. Nevertheless, the system needs at least two additional eccentric plates and only allows shift in horizontal and vertical directions.
In EP 1 248 131 a projection lens shifting mechanism for a projector that enables two-dimensional movement of a projection lens is described. The system comprises a projection lens fixed on a plate which can be moved in parallel with a second plate fixed to the remaining part of the projection system. For focussing or zooming, the projection lens is provided with a separate rotational system. The horizontal and vertical movement of the projection lens can not be performed completely motorized as levers are provided for switching the shifting mechanism between a shiftable state and a non-shiftable state. In EP 1 248 131 three plates are used for providing horizontal and vertical shifting possibilities for a projection lens. Thus the shifting mechanism has a relative complex structure, comprising multiple plates.
The above mentioned solutions for mechanisms for shifting the projection lens all need at least two plates for providing a shift perpendicular to the optical axis of the system. This makes these solutions relative complex and requires very high precision production of the plates, whereby small errors have a large influence on the quality of the shifting mechanism. To prevent the above mentioned systems from hysteresis effects, fixing means are often introduced to fix the position after adjustment has been applied. Furthermore, none of the known systems provide a way to extend the degrees of freedom for the motion to e.g. to Scheimpflug correction, based on their current shifting mechanism.